--> Lot of power-up twitch: Almost about 15 degrees. Is there a way to save power-down position? I think this is automatic in other standard Servos. --> It is turning lot more than 3.5 rotations. Setting to position-255, our assumption is, it should stop after 3.5 rotations. But is going lot more rotations!! --> Setting to position anything less than 10, does not stop the Servo. It appears it is running full speed to position 0, and then instead of stopping, it continue to rotate slowly.

Of all, power-up is twitch huge issue, it can not be used!!! We could be missing something here.. .. here the simple code to set to postion.

We haven't used HS-785 before so I can't explain why it is rotating more than 3.5 turns. The only comment I have is to make sure you did not have gears in between the servo and your mechanism. If you do, then you need to take into account the gear ratio. Regarding the "power up twitching". This is a side effect of the Tetrix servo controller initialization. I am not aware of any way to prevent that from happening. The twitching is very undesirable in some applications. But knowing that, we are always able to get around the problem by some clever mechanical design. For example, we used a servo to control a latch that locks the ramp from deploying in last year's competition. With the servo's power up twitch, it could unlatch and deploy the ramp even before competition starts. But we noted the direction of the twitch and designed the latch direction such that it will not unlatch in the direction of the twitch.BTW, we are using standard 180-degree servos this year. Since our application requires very accurate angle from the servo, so we plotted the servo angle against the range 0-255. This may help explaining some of your observations. Note that between the value 0 to 50, the servo did not really move until the value is greater than 50. Also it reaches the max angle at around the value 185 way before 255. But at least the angle is very linear in between that range.

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Sun Nov 10, 2013 4:17 am

Team4855

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Joined: Thu Nov 14, 2013 11:38 pmPosts: 4Location: Portland, OR

Re: Issues with HS-785 Quarter Scale Servo

We have observed similar behavior with our HS-785HB servers in that the range exceeds 4 rotations and seems to vary under different conditions. We tried the sample BasicServo program with different increments and the number of rotations seems to vary with different increments. The Hitec generic servo manual specifies the pulse duration required is .9 millisecond to 2.1 milliseconds with the center at 1.5 milliseconds. The Hitechnic Servo Controller Brief v2 says the pulse width range is .75 milliseconds to 2.25 milliseconds. We assume this pulse width variation is linear over the 0 to 255 servo position range. The pulse width slope is .05859375 milliseconds per count =(2.25-.75)/256. Therefore the servo position range should be restricted to 26 to 229 in order to stay within the .9-2.1 millisecond range. We don't recall twitching on power-up. We did a power-up test on a HS-785 servo and with a simple setup of an NXT with just a servo controller. The servo seems to come up inactive.

We have observed similar behavior with our HS-785HB servers in that the range exceeds 4 rotations and seems to vary under different conditions. We tried the sample BasicServo program with different increments and the number of rotations seems to vary with different increments. The Hitec generic servo manual specifies the pulse duration required is .9 millisecond to 2.1 milliseconds with the center at 1.5 milliseconds. The Hitechnic Servo Controller Brief v2 says the pulse width range is .75 milliseconds to 2.25 milliseconds. We assume this pulse width variation is linear over the 0 to 255 servo position range. The pulse width slope is .05859375 milliseconds per count =(2.25-.75)/256. Therefore the servo position range should be restricted to 26 to 229 in order to stay within the .9-2.1 millisecond range. We don't recall twitching on power-up. We did a power-up test on a HS-785 servo and with a simple setup of an NXT with just a servo controller. The servo seems to come up inactive.

Interesting..., I am not aware of the Hitechnic Servo Controller "Brief v2". Was there a v1? If so, what are the differences? I need to check ours and determine if they are v1s or v2s. The lego website (http://www.legoeducation.us/eng/product ... oller/1649) does not mention anything about versions. I wonder if the versions has anything to do with the twitching.

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